This invention relates to a continuous method for the precipitation of lignin from black liquor, in which method the pH of black liquor is lowered to the precipitation point of lignin and precipitated lignin is separated from black liquor. The invention also relates to an apparatus for the precipitation of lignin.
Lignin is the second most general biopolymer in the world after cellulose. Chemically, it is a heavily branched polyphenol that is an amorphous substance irregular in structure, closely bonded to wood fibers and difficult to separate in the processing of wood. In the preparation of chemical pulp, lignin is separated from wood fibers in the cooking of pulp, whereby it is dissolved in the cooking liquor. The black liquor obtained in sulfate cooking contains cooking chemicals, lignin dissolved from wood, and other substances dissolved from wood. When lignin dissolves in the sulfate cooking, its structure is also altered relative to the native lignin.
Being an organic biopolymer containing a great deal of carbon, lignin has a heating value, and the lignin remaining in black liquor has traditionally been burnt in the soda boiler of the pulp mill to produce heat and electricity. Lignin can also be separated from black liquor and used for the manufacture of different chemicals. As the synthesis methods are developing or new applications of use are being discovered, the importance of lignin as raw material in different products may grow, whereby lignin will be separated in a greater deal for further processing. Therefore, different methods for separating lignin from black liquor have been developed.
A general method for the separation of lignin is precipitation. Lignin remains dissolved within the heavily alkaline pH range of black liquor. When the pH decreases below 10, most of the lignin is precipitated as the phenolic hydroxyl groups it contains are being neutralized. Since the precipitation of lignin is based on the reduction of its solubility as the pH decreases, any pH decreasing acidifying agent, for example sulfuric acid or carbon dioxide, is used as a precipitation chemical to be added to black liquor. Since black liquor from which lignin has been precipitated is returned to the chemical circulation of the pulp mill, the use of acid generates substances interfering with the chemical balances of the circulation. Therefore, carbon dioxide is a widely used precipitation chemical that is led to black liquor in a gaseous state. Precipitated lignin can be separated from black liquor by filtering.
Known from international publication WO2009/104995 is a method for the precipitation of lignin, wherein lignin is precipitated from black liquor preferably by using carbon dioxide. Precipitation is typically followed by an “ageing”, or “maturing”, step, the intention of which is to raise the particle size of precipitated lignin before separating it by filtration, preferably pressure filtration. After filtration, lignin is washed by a filtrate obtained from a subsequent resuspension and filtration step, the ion strength and pH of the filtrate being adjusted if necessary.
Methods for the precipitation and separation of lignin from black liquor by acidification have also been disclosed in international publications WO2006/031175 and WO2006/038863.
All three patents focus on the filtration and washing of the obtained lignin and on the circulation of different solutions. In addition, it can be concluded that they use a batch reactor to precipitate lignin.
International publication WO2008/079072 discloses a method wherein lignin is precipitated from a black liqueur flow obtained from the evaporation plant by lowering the pH to below 10.5 using carbon dioxide. Precipitated lignin is washed in at least one step by a washing solution that includes calcium and magnesium ions to replace sodium so as to obtain lignin that has a low sodium content.
Known from U.S. Pat. No. 2,623,040 is a method wherein lignin is precipitated from black liquor by first leading combustion gases in a counterflow direction relative to the black liquor through two columns, after which the black liquor thus acidified is heated to a temperature of more than 75° C. and provided in a mild mixing by letting it flow through a tubular heater wherein the pressure is kept constant and above the vapor pressure corresponding to the temperature. The description of the patent discloses that black liquor is actually led via an externally heated coil. When black liquor thus processed is finally cooled, coagulated lignin is obtained that is easy to filter. It is particularly mentioned in the patent that the intensity of the mixing that is expressed as the Reynolds number affects the size of coagulated particles, and, according to tests, the Reynolds numbers of more than 2000 (corresponding to turbulence) resulted in such small particles sizes (1 to 7 μm) that lignin was practically impossible to filter.
Article R. Alén, E. Sjöström, P. Vaskikari “Carbon dioxide precipitation of lignin from alkaline pulping liquors” Cellulose Chemistry Technol., 19 537-541 (1985) discloses the processing of black liquor in an autoclave at a pressure of 800 kPa (approximately 8 bar), wherein black liquor samples were saturated with carbon dioxide so as to have a final pH of 8.7 after a 60 minute treatment. The lignin thus precipitated was separated by centrifugation. Carbonation carried out at a high pressure was discovered to increase the yield from black liquor. The employed device was a normal laboratory autoclave, wherein the pressure is slowly decreased before opening the autoclave and extracting the samples, and it is comparable to a batch process. The publication discusses factors affecting the precipitation of lignin and does not disclose an industrial process.
Article J. F. Howell, R. W. Thring “Hardwood lignin recovery using generator waste acid. Statistical analysis and simulation” Ind. Eng. Chem. Res., 39 2534-2540 (2000) models, on the basis of laboratory tests, the precipitation of lignin by means of waste acid obtained from the production of chlorine dioxide. In the conclusions, it is stated that, for the filterability of precipitated lignin, it is preferred to use as small a mixing rate as possible in the process.
The drawback of the batch process is the large reaction vessel it requires and the need to direct hydrogen sulfide separated from black liquor in connection with acidification away so as not to constitute a hazard. If carbon dioxide is used as the acidifying agent, which is preferred for the chemical balance of the pulp mill, acidification takes a lot of time in a normal-sized vessel. Reaction times for the precipitation of lignin are at least 30 minutes, after which there is yet the ageing step. Such a batch process does not allow for fast changes in the process conditions.
Continuous methods have also been disclosed, one example of which is the above-mentioned U.S. Pat. No. 2,623,040. However, to date, there has not been disclosed a method wherein particles sufficiently large for the separation of lignin, for example agglomerated particles, could be continuously produced with a short dwell time. After acidification, whether effected by carbon dioxide or acid, there has traditionally been the so-called “ageing” step where the particle size has been allowed to grow so as more easily to separate precipitated lignin from black liquor for later processing. This is also the case in the treatment disclosed in the above-mentioned U.S. Pat. No. 2,623,040 in the tubular heater after acidification. Such post-processing steps require additional process vessels and other devices and the dwell times become long.